A balun is a passive electronic circuit that can be used for conversion between symmetrical (balanced) and non-symmetrical (unbalanced) transmission lines.
At low frequencies, and less frequently at high frequencies, a variety of constructions are used to form baluns. For example, coaxial transmission line segments can be used to form baluns. A quarter wave length of coaxial cable having its outer conductor grounded at a single ended side, and an input applied to the single ended end of the quarter wave length cable will produce a balanced output between the cable conductors at the opposite end of the cable. A balanced signal applied to the non-grounded end will produce a single ended output at the grounded end.
Printed circuit forms of baluns have also been used. In U.S. Pat. No. 4,193,048 a balun transformer made from stripline elements formed on a printed circuit board is described. The balun transformer is fabricated from a pair of conductors each having first and second ends located on opposite sides of the printed circuit board. The first end of each conductor is located adjacent its second end.
U.S. Pat. No. 5,061,910 attempts to provide an improved printed circuit balun that includes a plurality of serially connected first conductor elements, preferably a contiguous merged conductor extending between a single ended signal port and ground, and a plurality of second conductor elements, also preferably in the form of a contiguous merged conductor coupled to the first conductor elements and electrically isolated therefrom, the second conductor elements extending in electrical symmetry from ground to a balanced port, the first and second conductor elements being separated by an electrical isolation layer, preferably the dielectric layer of the printed circuit board.
U.S. Pat. No. 5,697,088 describes a more recent configuration of stripline elements to form a balun useful at very high frequencies.
U.S. Pat. No. 5,644,272 shows a balun having both distributed (stripline) elements and discrete elements combined in a multi-layer dielectric structure.
Baluns including coaxial cable and wave guide, microwave circuits such as strip lines and micro strips, and other constructions are known to those skilled in the art. For the most part, known balun configurations are limited to certain specific impedance transformations such as one-to-one baluns at useful characteristic impedances such as 50 ohms and 75 ohms, two-to-one impedance transformations and the like. Teretofore,we believe that no method has been known for producing baluns having impedance transformation characteristics other than those certain values produced by those known configurations just mentioned. There is a need for baluns that match specific input and output impedances produced by transistor amplifiers, antenna splitters and combiners, and the like, that are not met by known balun constructions.
It is an object of this invention to provide a balun formed from a pair of symmetrical couplers, preferably symmetrical backward wave couplers, that can provide desired combinations of bandwidth and impedance transformation over useful ranges, so that substantially exact matching between balanced and unbalanced circuits can be produced.
It is another object of this invention to provide such a balun that can be implemented in a variety of forms including micro-strips and strip lines useful over a wide range of frequencies including microwave frequencies.
It is another object of this invention to provide a method for determining the characteristics, specifically the characteristic impedance and the normalized even mode impedance for symmetrical couplers to produce the desired combinations of band widths, operating frequency and impedance matching in a balun in accordance with the invention.
Briefly stated, and in accordance with a presently preferred embodiment of the invention, a balun includes first and second symmetrical couplers, preferably first and second backward wave couplers connected to form a balun having an unbalanced port and a balanced port. More specifically, a balun in accordance with this invention includes first and second backward wave symmetrical couplers each having an input port, a direct port, coupled port, and an isolated port in which the input port of a first coupler is connected to the unbalanced port of the balun, the coupled port of the first coupler is connected to an input port of the second coupler, and the isolated port of the first coupler and the direct port of the second coupler are connected to the balanced ports of the balun respectively.
In accordance with another aspect of the invention, the direct port of the first coupler and the coupled port and the isolated port of the second coupler are connected to ground.
In accordance with another aspect of the invention, the first and second symmetrical couplers are substantially identical.
A method in accordance with the invention for providing a balun having a desired unbalanced port impedance and a desired balance port impedance includes the steps of selecting a desired balanced port impedance; selecting a desired unbalanced port impedance; determining the achievable normalized even mode impedance for the type of couplers to be used in the balun; calculating f(Z0en) for the type of coupler used in the balun; calculating Z0m for the coupler and then fabricating the first and second symmetrical couplers defined by Z0en and Z0m.